Lufy is a young man searching for the legendary One Piece treasure. Encouraged
by a pirate named Slash, he wants to become the greatest pirate on the Grand Line. Problem is, he doesn’t have a ship.
Or a crew…

One Piece is a fun series, with a quirky animation style.
The flag flies from the mast in pretty much every episode--only after Lufy gets that pirate ship of his, natch--so
it's easy to get a good look. Toonami: Digital Arsenal has the opening sequence [http://www.toonamiarsenal.com/], under Downloads: Other Shows. The flag is all black, by the way--the
frame is merely a handy, nautical-theme visual aid, so that it may be distinguished from the background.